* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
    
}
body {
    overflow-x: hidden;
    background-color: #f2f2f2;
    color: #000;
}
.header{
    height: 75px;
    /* 固定定位跟父级没有关系, 以屏幕为准 */
    position: fixed;
    /* 固定的盒子应该有宽度 */
    width: 100%;
    top: 0;
    left: 50%;
    transform: translateX(-50%);
    min-width: 320px;
    max-width: 640px;
    background-color: #fff;
}
.search-bar {
    display: flex;
    padding: 10px 0 0;
    align-items: center;
}
.search-bar .logo {
    width: 30px;
    height: 30px;
    margin: 0 10px;
}
.search{
    position: relative;
    line-height: 33px;
    height: 35px;
    flex: 1;
    font-size: 15px;
    color: #666;
    padding-left: 35px;
    border: 1px solid #e0e0e0;
    border-radius: 5px;
}
.search img{
    position: absolute;
    left: 6px;
    top: 6px;
    width: 23px;
    height: 23px;
    
}
.user {
    margin: 0 10px;
    width: 30px;
    height: 30px;
}
.user img {
    width: 100%;
    height: 100%;
}
a {
    text-decoration: none;
}

.nav{
    margin: 0 20px;
    display: flex;
    justify-content: space-around;
    height: 30px;
    font-size: 13px;
    text-align: center;
    line-height: 30px;
    
}
.banner{
    padding-top: 75px;
    min-width: 320px;
    max-width: 640px;
    max-height:400px; 
    margin: 0 auto;
    
}
.banner img{
    width: 100%;
    height: 100%;
}
